1. Field of the Invention
The present invention relates to a resistor of the type including an insulating chip substrate provided with at least one resistive layer, a pair of upper electrodes connected to the resistive layer, and a protection coat enclosing the resistive layer.
2. Description of the Related Art
In a conventional chip resistor of the above-described type, the protection coat tends to be made higher at the center of the upper surface of the chip substrate in comparison with the upper electrodes. Due to this uneven surface configuration, the conventional resistor can suffer several drawbacks. For instance, the chip resistor may fail to be picked up by a suction collet when it needs to be transferred from one place to another. As another example, the protection coat may be broken by allowing the suction collet to come into contact with the projecting portion of the coat.
Further, the conventional chip resistor may suffer the corrosion and the resultant breakage of the upper electrodes when these electrodes are made from a conductive paste containing silver for its main ingredient (the paste is referred to as a “silver paste” hereinbelow). Specifically, the air surrounding the chip resistor may contain sulfur compounds such as hydrogen sulfide gas (H2S). Affected by the gas, the upper electrodes are corroded, whereby the electrical connection can be completely broken.
To address the above problems, Japanese Patent Application Laid-open No. H08-236302 and No. 2002-184602, for example, propose an arrangement whereby an auxiliary electrode is additionally formed on each of the upper electrodes in a manner such that the auxiliary electrode extends onto part of the protection coat. (Thus, the contact portion between the auxiliary electrode and the protection coat is located above the upper electrode).
With this arrangement, the protection coat can be generally flush with each of the two-layered electrodes (i.e., the upper electrode and the auxiliary electrode), or the difference in height between the coat and the electrodes is made smaller. Accordingly, the chip resistor can be more easily picked up by a suction collet, and further, the corrosion of the upper electrodes due to the sulfur compounds in the air can be prevented since the upper electrodes are hidden under the auxiliary electrodes.
However, the teachings of the two Japanese patent applications mentioned above have been found ineffective in preventing the corrosion in the upper electrodes. According to the teaching of JP H08-236302, the auxiliary electrodes are made from a silver paste. Thus, the corrosion due to the airborne sulfur compounds will occur at the contact portion between the auxiliary electrode and the protection coat. Eventually the corrosion expands to damage the upper electrode.
According to the teaching of JP 2002-184602, on the other hand, the auxiliary electrodes are made not from a silver paste but from a nickel paste. In this case, the problem is that the contact portion of the auxiliary electrode with the protection coat is relatively thin, and therefore tends to be broken easily. When breakage occurs in the auxiliary electrode, the airborne sulfur compounds can penetrate through it, and corrodes the silver-containing upper electrode below.